This idea has been borrowed from http://afaithfulattempt.blogspot.co.nz/2011_07_01_archive.html.
Check out "A faithful attempt" blog as her instructions are very straightforward.
I used this last year with an older group of children. This group had more time. We used water colour paint in contrast to the coloured pencil we used last time. This offered more possibility to include detail. These water colour images are stronger and clearer.